Multi-station communication systems typically reside in one of two major categories, master controlled systems and master-less systems. The former category utilizes a central processing unit or other supervisory control which observes and dictates which station has access to the commonly shared data transmission resource; typically the bus interconnecting the various stations. Unlike such systems, the present invention is a master-less communication system.
With respect to master-less communication systems, two general categories exist; namely, contention systems and token pass systems. The former contention systems are unlike the present invention in that they allow multiple stations to contend for access to the bus as the general means for transferring control of the bus from one station to another station. The present invention, although allowing for the possibility of contention when multiple stations simultaneously demand access to the token, generally only uses a contention-less token pass system.
As to contention systems, the joint project developed by Digital Equipment Corporation, Maynard, Mass.; Intel Corporation, Santa Clara, Calif., and Xerox Corporation, Stamford, Conn., known as Ethernet.TM., and as described in U.S. Pat. No. 4,063,220, Metcalfe et al, allows any station connected to the bus to transmit information provided that the bus is clear just prior to and during the transmission. If it detects noise on the bus while transmitting, it is assumed that an interference or collision has occurred between itself and at least one other station attempting to transmit information on the bus at the same time. In order to correct this problem the transmitter section is disabled on each station and a random number generator is used to select an interval of time at the completion of which the next attempt at transmission will take place. At the same time, a counter counts the number of interferences, or collisions, which occur in the attempted transmissions of one data packet and weights the mean of the random number generator accordingly. In this way, the stations attempting to transmit will reattempt to transmit at different times and thus, in time, eliminate the contention between the stations.
The present invention does not utilize such a contention or collision detection system but rather uses the concept of a token which is passed from one station to another such that each station when owning the token has, during that time, the exclusive right to initiate high level message transmissions as well as to request return high level messages from any other station.
With respect to a token pass communication system, a representative prior art technique is disclosed in U.S. Pat. No. 4,058,681, Imaizumi et al. This reference discloses an information transmission system utilizing a token which is transmitted from one station to the next by use of a command established signal (ELS) and a transfer command signal (SEL). The token is passed from station to station through use of the SEL signal giving the address of the next station which is to have the token. Upon receiving the token, that next station transmits the command establish signal to inform the other stations that it does have command. If the station presently having the token does not transmit an ELS or SEL signal within a predetermined time, another station takes the token by transmitting its own ELS signal. A failure of a station to detect the transfer of the token from itself to the next addressed station within a predetermined time causes the station which originally had the token to retransmit a new SEL signal with a new address so as to transfer the token to a different station. This reference however does not disclose the concept of an arc of control wherein the present token owner (holder) knows both the station from which it received the token as well as the station to which it is to next transfer the token. This arc of control in combination with a demand window by which the present token owner can transfer the token to a demander not in the token list provides the capability of both patching in and patching out stations to the token list of stations presently passing ownership of the token on a systematic basis. Imaizumi et al also neither discloses nor suggests the concept of automatically changing the handshaking protocol between token passing stations.
Imaizumi et al further does not disclose or suggest the binding concept of the present invention wherein stations may have physical connection to the bus and yet never have the possibility of obtaining the token for control of the bus. These stations are thus only able to respond to other stations which can be token owners. The binding concept allows the communications system to interconnect with relatively low level access stations which would otherwise be incapable of transmitting information on the bus.
Furthermore, the present invention through use of its underlying perceived events and their causal effect through rule interaction to change station states, can be reconfigured to form new and different communication systems having different access mechanism protocols depending upon the particular requirements of a communication network desired by the user.